The title of Brigadoon, the 1947 musical by Lerner and Lowe, refers to a mythical Scottish town that appears for only a single day every 100 years. Though not quite as seldom seen as the town, Brigadoon the musical — the first hit by the team that later created the legendary shows My Fair Lady and Camelot — hasn’t had a Broadway revival in more than three decades. But it has been given a big new production, now extended through Aug. 17, at Chicago’s Goodman Theatre.